1. Field of the Invention
The present invention relates to a measuring apparatus and a measuring method for pattern dependent jitter, and in particular, to a measuring apparatus and a measuring method for pattern dependent jitter, in which a technique for measuring pattern dependent jitter which arises in dependence on a pattern of a data signal among jitter components included in the data signal.
2. Description of the Related Art
In a data transmission system, when fluctuation (jitter) in the phase of the data signal is large, the data signal cannot be normally transmitted.
Therefore, it is necessary to measure in advance the jitter characteristics of the data transmission system and equipment configuring the system.
FIG. 10 shows a configuration of a conventional jitter measuring apparatus 10 used for such an object.
In FIG. 10, a clock generating unit 11 outputs a clock signal CK having a predetermined frequency, and a pattern generating unit 12 outputs a data signal Dt which is synchronized with the clock signal CK, and which has a predetermined pattern having a predetermined bit (N) length.
A measuring object 1 is, for example, a clock/data recovery circuit used for a data transmission system.
The clock/data recovery circuit serving as the measuring object 1 outputs the data signal Dt from the pattern generating unit 12, as a data signal Dr.
The data signal Dr outputted from the measuring object 1 is inputted to a waveform observing device 13 as a data signal to be measured.
The waveform observing device 13 displays waveform of the inputted data signal Dr to be measured as will be described later.
Namely, the waveform observing device 13 divides the clock signal CK by P (here, P is a number less than the data length N of the data signal Dt), and displays so as to overwrite waveform of the data signal Dr to be measured due to the level displacement timing of the divided signal being made to be the trigger timing.
At this time, when a pattern of the data signal Dt is random, for example, as shown in FIG. 11, waveform in which a rise and a fall intersect is displayed on the waveform observing device 13.
This waveform is called eye-pattern, and when jitter in the data signal Dr to be measured is large, a width W of the intersection of the rise and the fall of the eye-pattern is made large.
Accordingly, a jitter amount of the data signal Dr to be measured can be grasped in accordance with the width W of the intersection of the eye-pattern displayed on the waveform observing device 13.
Note that a method for determining jitter by observing the eye-pattern of a data signal as described above is described in, for example, Patent document 1 (Jpn. Pat. Appln. KOKAI Publication No. 5-145582).
However, in the method for measuring jitter based on the width W of the eye-pattern displayed on the waveform observing device 13 as described above, pattern dependent jitter which arises in dependence on a pattern of a data signal cannot be grasped.
Namely, as jitters, there are random noise jitter arising due to noise of equipment themselves, external noise, or the like, and pattern dependent jitter arising due to a pattern of a data signal which is transmitted.
The pattern dependent jitter is jitter arising due to waveform distortion generated because a DC component cannot pass through when a data transmission passing band of the measuring object 1 is high (several GHz), duty cycle distortion of a data signal, waveform distortion generated because the frequency characteristic of the measuring object 1 is not sufficient for a frequency of the signal which is transmitted, or the like.
This pattern dependent jitter is not a serious problem when a data signal has strong random characteristic such as a pseudo-random pattern.
However, in a case where the data signal having the predetermined pattern is a data signal in which an unscrambled specific pattern always exists at the head position, such as a frame actually used for data transmission, for example, an SDH frame or a SONET frame, large pattern dependent jitter arises at the frame intervals (for example, 125 μs intervals).
Moreover, because the frequency of the pattern dependent jitter arising at this frame intervals is generally within a frequency band stipulated by jitter measurement, the pattern dependent jitter cannot be measured in distinction from other random noise jitter.
Further, in measurement of jitter in dependence on a pattern of a data signal as described above, it is necessary to exactly grasp the relation between the position of data and jitter.
However, it is difficult to exactly grasp the relation in the eye-pattern observation as described above.
Therefore, in the measurement of pattern dependent jitter, realization of a measuring apparatus for pattern dependent jitter and a jitter measuring method which can exactly grasp the relation between the position of data and jitter has been strongly desired.